Monotremes are really weird. Everything about them seems to have been thought up past some kind of deadline. Take the method by which they reproduce: after mating, the female lays an egg (echidna) or two (platypus). Then she carries them around until they hatch. Monotremes are mammalian, even if what they have is a kind of free demo version of mammalianism without the really useful features like live birth, so they lactate. But they have no nipples. The milk just leaks right out of glands in their skin, and the baby monotreme laps it up with sweeps of its tiny bill. [...]

The echidna is therefore "born" three times -- once as an egg, once when the egg hatches, and once when the puggle is evicted from the pouch and hidden by its mother. It's a good thing placentalism came along, or we'd all have to go through something like this. [...]

Echidna mating is mysterious and primordial. It is also rarely observed, but the following seem to be the basics. It begins when the female goes into estrus. Males, usually three or four of them, but sometimes as many as eleven, start following her around in a long single-file line called an "echidna train" (or even "echidna love train"). It seems very civilized, though it can go on for as long as six weeks. [...]

The male's four-headed penis, which he does not use to urinate, emerges only during the act of mating; the rest of the time he is indistinguishable from a female echidna, as his testicles are also inside his body.

Scientists from the Australian National University have proved what many have thought for years - platypuses are really weird.

In the international Nature journal today they report a platypus has five chromosones determining sex, not one - like the rest of the species in the world.

Professor Jennifer Graves says platypus have five X and five Y chromosomes, and when sperm are made it gets even stranger.

"What we've discovered is that these five Xs and five Ys line up in a great big long chain, that go XY XY XY XY XY XY, and then all the X chromosomes move to one pole, and all the Y chromosomes move to the other," she said.

Professor Graves says there is another unexpected finding.

"One end of the chain looks like human sex chromosomes but the other end of the chain looks like bird sex chromosomes, so the chain is actually linking a very ancient system of sex determination in birds and probably reptiles too," she said.